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Next time you strip the bolt, have a look at the firing pin flange to make sure that it's not interfering with the corresponding recess inside the bolt. Sometimes the flange can become peened or malformed over a little bit that it interferes with proper function. Increased friction from the dispersed metal around the flange to internal bolt safety lugs and then combined with cartridge fit between the different types of ammo could maybe be a possibility for part of the problem.
Also the cocking piece should be a nice fit on the tail of the firing pin and not overly tight. If it's binding, on the tail of the FP or when the cocking piece itself travels forward it could slow the pin down a small bit as it travels through the bore of the shroud. Could be some hardened or caked grease/oil lurking around those areas too.
I checked it last time I took it apart. Firing pin is nice and straight with very little wear marks. Thanks.